The present invention is equally applicable to the productive reclamation of normal soils which have been contaminated by brine spillage or duming from industrial operations, and to native soils exhibiting sodium contamination including, without limitative intent, such native soils as are commonly referred to as sodic, saline, saline-sodic, saline-alkali, solonetzic, solonchaks, etc.
When normal soils are contacted by sodium-rich waters, the sodium ions exchange cations from the exchange complex of the soil, i.e. the reactive clay and humic fractions, by ion exchange. Subsequent exposure to fresh water, for example as a result of natural precipitation, causes the clay aggregates of the soil to hydrate and swell until individual clay particles separate from the aggregates and become dispersed in the pore system of the soil. When this occurs, the movement of air, water and nutrients are restricted, thus rendering the soil less productive or even totally unproductive, in terms of vegetative growth, dependent upon the severity of the sodium contamination. Normal soils rich in reactive clays such as montmorillonites can thus be particularly adversely affected upon sodium contamination and subsequent hydration and dispersion of the clay aggregates.
A variety of methods involving the chemical treatment of soils with calcium salts, or other di- or poly-valent cation-containing compounds, have historically been used in attempts to correct sodium-induced dispersion of clay aggregates. As well, the cultivation of salt tolerant crops and physical treatments like leaching and drainage are at times used to assist in removal of sodium ions from sodium contaminated soils while providing some degree of agricultural productivity during reclamation.
The most commonly used chemical for treatment of sodium contaminated soils has traditionally been gypsum, due both to availability and low cost. Conventionally, the gypsum is applied to the surface of the soil and then incorporated by employing normal cultivation practices. Eventually the gypsum is dissolved by the native soil moisture, by irrigation water or by natural precipitation, or a combination thereof. Calcium ions thereby brought into aqueous solution displace sodium ions from the exchange complex of the soil by ion exchange as the calcium-enriched water percolates through the soil. The sodium ions replaced by calcium ions are then removed with the soil water through drainage, whether by natural drainage or artificial drainage systems. The enrichment of the soil with the calcium ions, that is the exchange of the sodium ions by calcium ions, causes the dispersed clay particles to reaggregate whereby the physical structure of the soil becomes more conducive to movement of the air, water and nutrients necessary for healthy vegetative growth.
There are however a number of disadvantages which restrict the use of gypsum for the reclamation of sodium contaminated soils. By way of example, gypsum has only a low water solubility, such that the concentration that can be dissolved in the soil water may not be sufficient to permit exchange of enough sodium ions to cause reaggregation of deeper subsoils because most of the calcium ions are depleted from the soil water prior to any substantial downward percolation. Since sodium ion replacement by calcium ions is most efficient when the calcium ion concentration is equal to or higher than the sodium ion concentration, a point is thus reached where the deeper penetrating soil waters do not contain sufficient calcium ions for further exchange of sodium ions. Accordingly, the treatment of deeper, sodium contaminated subsoils through the use of gypsum is inefficient or even impossible in cases of severe sodium contamination. Additionally, to maximize the amount of calcium that can be dissolved in the soil water, the gypsum should be incorporated into the soil. This however is often difficult or even impossible with sodium-induced dispersed soils since such soils frequently retain sufficient moisture, i.e. tend to remain substantially water-logged, that conventional cultivation equipment can not be effectively operated.
In efforts to overcome the disadvantages associated with the use of gypsum, the effectiveness of more soluble calcium salts, such as calcium chloride and calcium nitrate, has been investigated by a number of researchers. It was however discovered that the use of highly soluble calcium salts per se was not particularly efficient, due apparently to rapid loss of calcium ions. In this regard, reference may be had to DE JONG, E. 1982. Reclamation of soils contaminated by sodium chloride. Can. J. Soil Sci. 62:351-364, wherein the author stated, on Page 361:
It appears that the high solubility of Ca(NO.sub.3).sub.2 was a disadvantage as it allowed a rapid loss of Ca. PA1 Surface-applied gypsum and highly soluble amendments such as Ca(NO.sub.3).sub.2 were much less efficient than incorporated gypsum. PA1 (a) preparing an initial aqueous solution having a concentration in the range of from about 1 to 4% by weight of a water soluble calcium or magnesium salt and dissolving therein from about 1 to 2% by weight of a water soluble polymer having an average molecular weight of at least about 1,000,000; PA1 (b) preparing a substantially saturated aqueous solution of the water soluble calcium or magnesium salt employed in step (a); and PA1 (c) mixing from about 5 to 20% by volume of the polymer-containing solution of step (a) with from about 80 to 95% by volume of the substantially saturated solution of step (b) to obtain the stable, concentrated solution. PA1 (a) preparing an initial aqueous solution of calcium nitrate having a concentration in the range of from about 1 to 2% by weight and dissolving therein from about 1 to 2% by weight of a water soluble polyacyrlamide polymer having an average molecular weight of at least about 1,000,000; PA1 (b) preparing a substantially saturated aqueous solution of calcium nitrate; and PA1 (c) mixing from about 5 to 20% by volume of the polymer-containing solution of step (a) with from about 80 to 95% by volume of the substantially saturated solution of step (b) to obtain the stable, concentrated solution. PA1 (a) preparing an initial aqueous solution of calcium nitrate having a concentration of about 2% by weight and dissolving therein from about 1 to 2% by weight of an anionic polyacrylamide polymer having an average molecular weight in the range of from about 10,000,000 to 20,000,000; PA1 (b) preparing a substantially saturated aqueous solution of calcium nitrate; and PA1 (c) mixing from about 5 to 10% by volume of the polymer containing solution of step (a) with about 90 to 95% by volume of the substantially saturated solution of step (b).
. . and concluded, on Page 363:
Another approach to the chemical reclamation of sodium contaminated dispersed soils has been to apply organic matter in an attempt to improve the physical, i.e. aggregated, structure. Many types of naturally occurring sources of organic matter including manure, straw, hay and sewer sludge have been tried and, while to some extent effective, are not particularly well suited for large scale applications because of problems of availability and associated handling costs.
Hedrick et al U.S. Pat. No. 2,625,529, issued Jan. 13, 1953, and related Hedrick et al Canadian Patent No. 625,135, issued Aug. 8, 1961, teach the improvement of the physical structure of non-sodium contaminated soils by the application of synthetic polyelectrolytes. A number of classes of suitable water soluble polymers are disclosed, effective members of which are said to have a maximum average molecular weight in the order of about 100,000. The polymers are either applied in dry form to the soil surface and then incorporated by cultivation or are sprayed onto the soil surface in the form of aqueous solutions, which may additionally include plant nutrients such as mineral fertilizers. Polymer application rates ranging from 0.001 to 2.0% by weight of the tillable top soil are said to be required, with optimum results reputedly being obtained by the use of from 0.01 to 0.2% by weight of the tillable top soil. Polymer application rates at less than 0.005% by weight of the tillable soil are not however exemplified and, indeed, it seems from the few comparative examples employing a polymer application rate of 0.005% by weight of the tillable soil that higher application rates are apparently more effective.
It has been reported (ALLISON, L. E. 1952. Effect of synthetic polyelectrolytes on the structure of saline and alkali soils. Soil Sci. 73:443-454; and MARTIN, W. P., ENGIBOUS, J. C. and BURNETT, E. 1952. Soil and crop responses from field applications of soil conditions. Soil Sci. 73:455-471) that the application of approximately 0.1% on a dry soil weight basis of the types of synthetic polyelectrolytes generally taught in the aforementioned Hedrick et al patents to the soil surface, followed by mechanical mixing, achieved aggregation of alkali, i.e. sodium-rich, soils. However, economic considerations appeared to preclude large scale applications.
It has also been reported (CARR, C. E. and GREENLAND, D. J. Proceedings Symposium On The Fundamentals of Soil Conditioning, Ghent, Apr. 17-21, 1972, Editor: M. DeBoodt) that high molecular weight polymers, those having an average molecular weight in excess of 70,000, are more effective than low molecular weight polymers in promoting aggregation. However, when such high molecular weight polymers are prepared in aqueous solutions containing a sufficient concentration of polymer to promote effective aggregation, the resultant viscosities are so high that diffusion of the polymer solutions into the soil is severely restricted.